A few thoughts from John Fleck, a writer of journalism and other things, living in New Mexico

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Leap Year Bias

The addition of one extra day in February normally every fourth year produces a significant seasonal drift in the monthly values of that year in four major temperature datasets used in climate change analysis. The addition of a ‘leap year day’ for the Northern Hemisphere creates statistically significantly colder months of July to December and, to a lesser degree warmer months of February to June than correspondingly common (non-leap year) months.

This seems to be of no great import in discussions of long term trends, but it’s a clever bit of work.